The present invention relates to methods and apparatus for recovering solvents and more particularly to methods and apparatus for economically and safely recovering such solvents with minimum energy requirements and capital costs.
The curing of solvent borne coatings applied to materials has been widely practiced for many years in different industries. Typically, materials bearing such coatings are passed through an oven at a temperature of approximately 300.degree. F. and during the curing process, such solvents are evaporated. As such solvents are typically comprised of hydrocarbons such as hexane, octane, or methyl ethyl ketone, the evaporation thereof into an oven atmosphere has resulted in potentially explosive conditions. One technique for rendering ovens safe during such curing processes has been to sweep large volumes of air through the oven thereby maintaining the solvent vapor concentration therein well below (25% or less) the lower explosive limit (LEL) and consequently avoiding explosive conditions in such ovens. In the past, it has been common practice to simply exhaust the foregoing oven atmosphere to the ambient atmosphere. However, recent environmental regulations have required that solvent be largely eliminated from such exhaust and in practice, this has led to the use of exhaust incinerators. It has been found that such techniques for rendering curing ovens safe have consumed excessive quantities of energy due to the fact that extremely high air flows through the oven tend to remove heat therefrom and thus require substantially greater quantities of energy (heat) in order to maintain a predetermined temperature therein. In addition, as the oven atmosphere supplied to an incineration device is relatively dilute in solvent vapor, natural gas or other combustible materials are generally added to the exhaust in order to incinerate substantially all of such solvent vapors. Although the use of certain instrumentation has been effective in enabling slightly higher solvent vapor concentrations, yet still well below LEL levels and less than 50% of the LEL as described in Industrial Finishing, March 1977, Pg. 16-18, air dilution techniques are still relatively inefficient with respect to energy usage and do not result in the recovery of solvent for further use.
Another technique in which a degree of solvent recovery is effected is illustrated in U.S. Pat. No. 4,012,847. In this system, relatively high air flows are supplied to a curing oven and the atmosphere containing solvent vapor is withdrawn therefrom and passed to a chiller to remove a portion of such vapor by condensation. The chiller exhaust gas is supplied through a circulating system including an adsorber which in turn is effective to condense or separate out further quantities of such vapors with an air stream being circulated through the adsorber and subsequently returned to the curing oven. Although the technique described in this reference results in some degree of solvent recovery, this system suffers from the aforementioned disadvantages of excessive energy consumption.
In order to remedy one or more of the defects of the aforementioned air dilution type solvent recovery systems, it has been proposed to introduce an inert gas into an oven during curing operations. A system utilizing an "inert" gas is illustrated in U.S. Pat. No. 3,909,953 wherein an oven atmosphere containing solvent vapor is withdrawn and passed to an incinerator device which is supplied with air and natural gas for enabling incineration of such solvent vapor. The effluent of the incineration device which contains less than 5% oxygen is subsequently returned as an "inert" gas to the oven thereby supplying the heat requirements of the oven without use of any additional fuel. This system obviates the need for dilution air and enables solvent vapor concentrations above the lower explosive limit to be maintained in an oven as well as satisfying hydrocarbon emission standards by passing only relatively "clean" gases to atmosphere. However, such a system is nonetheless complex and costly due to the incineration equipment required and is relatively expensive to operate in that the solvent vapor is utilized as a fuel in the incineration device rather than being recovered for eventual reuse. This combustion of solvent vapor yields considerably more heat than is required to establish required oven curing temperatures. Consequently, a large portion, e.g. two-thirds, of the incinerator heat must be wasted or "dumped" to ambient atmosphere.
It is also known from U.S. Pat. No. 2,746,168 to supply an "inert" gas to a drier, to remove the drier atmosphere containing solvent vapors and to condense such vapors to enable recovery of solvent. However, this type of system relies upon the recirculation of inert gas which tends to reduce the partial pressure of solvent vapor in the drier and hence the efficiency of the condensation device and therefore solvent recovery. Furthermore, such a system also requires the recycled "inert" gas to be heated and, as this inert gas is specifically described as the combustion products of natural gas and air, substantial quantities of hydrocarbon materials are nonetheless required for such a recovery system. A similar solvent recovery system is described in U.S. Pat. No. 3,798,787 and although this reference alleges lower fuel, power and refrigeration costs by utilizing a dry carrier or diluent gas, a complex arrangement of mechanical baffles is required to provide entry and exit air locks.
Accordingly, the prior art exhibits a clear need for a hydrocarbon solvent recovery technique in which the consumption of fuel sources and environmentally undesirable emissions are minimized by means of relatively simple, conventional and inexpensive equipment.